f(θ)=|AB|*|BC|cos(2π/3)=-|AB|*|BC|/2,
根据正弦定理,
|BC|/sinθ=|AC|/sin(2π/3),
|BC|=2√3sinθ/3,
|AB|/sin(π-2/3π-θ)=|AC|/sin(π/3-θ),
|AB|=2√3sin(π/3-θ)/3,
f(θ)=-(2√3sinθ)*2√3sin(π/3-θ)/9
∴f(x)=-4sinθsin(π/3-θ)/3.
f(θ)=|AB|*|BC|cos(2π/3)=-|AB|*|BC|/2,
根据正弦定理,
|BC|/sinθ=|AC|/sin(2π/3),
|BC|=2√3sinθ/3,
|AB|/sin(π-2/3π-θ)=|AC|/sin(π/3-θ),
|AB|=2√3sin(π/3-θ)/3,
f(θ)=-(2√3sinθ)*2√3sin(π/3-θ)/9
∴f(x)=-4sinθsin(π/3-θ)/3.